let list = new LinkedList()
        list.append(1)
        list.append(2)
        list.append(3)
        list.append(4)
        list.append(5)
        console.log( list.indexOf(8));
        console.log( list.indexOf(7));
        console.log( list.indexOf(6));
        console.log( list.indexOf(-1));
        console.log( list.indexOf(5));
        console.log( list.indexOf(5));
        console.log( list.indexOf(4));
        console.log( list.indexOf(3));
        console.log( list.indexOf(2));
        console.log( list.indexOf(1));
